As is known, many varieties of data storage devices (e.g. disk drives, floppy drives, and optical drives including CD, DVD, and Blu-Ray) are used to provide data storage for a host device, either directly, or through a server. The server may be available through a network, such as but not limited to a local area network (LAN), wide area network (WAN), or the Internet, to provide storage area network (SAN) or network attached storage (NAS) Typical host devices include stand-alone computer systems such as a desktop or laptop computer, enterprise storage devices such as servers, storage arrays such as a redundant array of independent disks (RAID) arrays, storage routers, storage switches and storage directors, and other consumer devices that use magnetic and optical storage systems such as video game systems and digital video recorders. These devices provide high storage capacity in a cost effective manner.
Many different communication channels are available. Communications channels allow wired or wireless communications for the transmission of audio, video and data. These wired, wireless and optical communication channels may include fiber optics, laser based communications, satellite based communications, cellular communications, cable communications, radio frequency (RF) and traditional wired and wireless communications. These communications allow for the delivery of video, Internet, audio, voice, and data transmission services throughout the world. By providing communication channels with large bandwidth capacity, communications channels facilitate the exchange of information between people in an ever shrinking global environment.
The structure and operation of hard disk drives is generally known. Hard disk drives include, generally, a case, a hard disk having magnetically alterable properties, and a read/write mechanism including Read/Write (RW) heads operable to write data to the hard disk by locally alerting the magnetic properties of the hard disk and to read data from the hard disk by reading local magnetic properties of the hard disk. The hard disk may include multiple platters, each platter being a planar disk.
All information stored on the hard disk is recorded in tracks, which are concentric circles organized on the surface of the platters. FIG. 1 depicts a pattern of radially-spaced concentric data tracks 102 within a disk 100. Data stored on the disks may be accessed by moving RW heads radially as driven by a head actuator to the radial location of the track containing the data. To efficiently and quickly access this data, fine control of RW hard positioning is required. The track-based organization of data on the hard disk(s) allows for easy access to any part of the disk, which is why hard disk drives are called “random access” storage devices.
Each track is broken down into subunits of sectors. Sectors, in turn, are composed of some number of bytes. This number of bytes can vary greatly from device to device, but is typically (although not necessarily) constant within a given device. The hierarchy of storage units (tracks, sectors, bytes) provides design flexibility. Each sector contains user information that has been encoded to facilitate retrieval of the information. The encoding is performed in many steps and these steps vary from device to device, but again, are typically constant within a given device. The encoding steps include, but are not limited to addition of a preamble (bits prepended to the data sector to facilitate determination of the phase of the analog data signal from the disc), a sync mark (bits placed between the preamble and encoded user data to identify the start of the encoded user data), modulation coding (any scheme that makes the user data easier to read by the reading mechanism of the device; this type of encoding “transforms” the bit sequence, often making it substantially different in appearance from the original user data), and error correction coding (bits appended to the encoded user data that allows correction of some limited number of errors that may have occurred during retrieval). The main point is that several common (but variable) procedures are invoked to improve the robustness and readability of the data stored in the device.
Within such hard disk drives (HDDs), disk drive controllers control the various processes associated with the read/write of data to the physical media. As the amount of data stored to the physical media increases, the ability to accurately read data from the physical media is adversely effected. One factor affecting the ability to accurately read this data is inter symbol interference (ISI). ISI is the process by which nearby symbols interact with each other in a detrimental way. Although discussed here as a problem associated with HDDs, this problem may be present within any communication channel.
To allow higher storage within physical media such as that of a hard disk drive (HDD), one solution in telecommunications and data storage has been to intentionally write symbols close together and utilize the Viterbi algorithm (or any other sequence detector) and knowledge of how the symbols interact to recover the bit sequence from a noisy analog signal. When applying this solution, the data interferes in a controlled manner and additionally becomes distorted by noise and/or other interfering signals. This noise and interfering signals must be overcome in order to properly read back the pattern of “1's” and “0's” correctly. The Viterbi algorithm is an efficient dynamic programming algorithm that finds the most likely sequence of bits received by comparing a received sequence of points sampled from the analog read back waveform to every possible sequence of bits transmitted. This best sequence is referred to as the “best path through the trellis.” The trellis tracks all possible paths and consists of states, which help track the bit decisions associated with the path through them. Other techniques design symbols that are more robust against ISI. Decreasing the symbol rate (the “baud rate”), and keeping the data bit rate constant (by coding more bits per symbol), reduces inter symbol interference.
A sizable market has developed for these data storage devices and the price per unit of storage has steadily dropped. A similar market has developed for high capacity communication channels. As increased capacity within communication channels and data storage devices are provided, the need to retrieve data from these communication channels and data storage devices with greater accuracy grows.